1. Field of the Invention
This invention relates generally to a sealing process and an apparatus therefor and, more particularly, to a method and apparatus for hermetically sealing ceramic or metal covers to ceramic packages with a vitreous sealant.
2. Description of the Prior Art
Many electronic circuits and components are packaged in sealed containers to protect them from possible damage due to exposure to the surrounding environment. Sealants employed in the past are (1) metallic sealants such as solders; (2) ceramics including various glasses; and (3) organic sealants such as expoxies. Of these, only the first two will result in a truly hermetic seal since the organic sealants will become permeable to large quantities of matter in a relatively short period of time (e.g. days) when compared to the life of the circuit housed within. If the sealant became permeable to, for example, water vapor, a chemical reaction could take place with certain ions within the package such as chlorine, sodium and potassium to produce corrosive products which would in turn attack wiring and/or the electronic components and hasten failure of the packaged circuit.
Several metal alloys have been employed as sealants with varying results. One very desirable property of a metallic sealant is its ability to withstand high temperatures. For example, ordinary lead-tin solder provides satisfactory seals at temperatures up to 125.degree. C. For temperatures at say 150.degree. C., an alloy of 80 percent gold and 20 percent tin is preferable although obviously expensive. Unfortunately, the melting point of this alloy is approximately 280.degree. C., and fusing the material during the sealing process may heat the electronic components possibly resulting in undesirable parameter shifts and premature failure of the circuit.
The danger of circuit or device failure due to high sealing temperatures is even more acute when vitreous sealants are employed since such sealants have fusing temperatures above 400.degree. C. While vitreous sealants are much less expensive than the previously discussed gold-tin alloy, the higher fusing temperatures have precluded the use of such glass sealants in cases where component heating is a problem.
Methods have been developed for sealing packages without unduly heating their contents; i.e. impulse or impact welding using either fixed or rolling electrodes. These techniques require metal covers and packages or, at the very least, metal sealing surfaces on both the cover and package. Unfortunately, the equipment needed to carry on such welding techniques is very expensive, and special tooling is required for each package size. The metal packages themselves are also relatively expensive.
The term "hot-capping" refers to a method for sealing either metallic or ceramic lids and packages using a metallic sealant. If either the lid or package is ceramic, it must have a metallic sealing surface fused thereto. The seal is effected by applying an electrically heated surface to the cover. However, since the cover must be heated to at least the sealing temperature, the temperature of the electronic component or circuit will be raised.
Since ceramic packages are less expensive than metallic packages, and since vitreous sealing materials are less expensive than metal based sealants, the most economical approach would be to use ceramic packages and covers and seal them with a vitreous sealant without requiring fused metallization sealing layers.